Telecommunication networks must be properly maintained to ensure that adequate network performance is achieved and that end-user services are supported. Maintenance functions include "performance management" (continuous in-service performance monitoring for proactive warning of performance degradation) and "fault management" (detection and location of network trouble and failure).
Delay monitoring is important in managing performance of ATM or other telecommunications networks and the following parameters are used for such purposes because they affect important network management functions.
Cell Transfer Delay (CTD)
Relates to throughput and response time for high speed data services, and is used for:
provisioning congestion and protocol parameters such as window sizes and time-outs; PA1 selecting low delay routes (e.g. to avoid satellite links); and PA1 deploying echo cancellers. PA1 dimensioning AAL-1 buffers for smoothing CBR (continuous bit rate) traffic; PA1 detecting excessive traffic; and PA1 predicting congestion. PA1 "To measure cell delay variation, the following actions have to be performed: PA1 a) Near-End monitoring which provides performance of a received signal from its origination to its termination. Bit Interleaved Parity (BIP) is used for ATM by forward monitoring OAM cells. The monitoring point is at the received signal termination. PA1 b) Far-End monitoring provides performance of a transmitted signal from its origination to its termination. For ATM, performance at the far-end termination is sent back to the monitoring point in received signal overhead, e.g. backward reporting OAM cells. The monitoring point is at the received signal termination where the overhead is read. PA1 c) Intermediate monitoring is at intermediate locations in a transparent mode such that near- and far-end performance indicators are read but not terminated. This provides performance of the received signal from its origination to the intermediate monitoring point (e.g. by calculating BIP in forward monitoring OAM cells), and performance of a transmitted signal from its origination to its termination (e.g. by reading backward reporting OAM cells at the intermediate monitoring point).
Cell Delay Variation (CDV)
Used for:
To support performance and fault management functions of VPC/VCC (virtual path connection/virtual channel connection) in ATM networks, OAM cells are defined to carry operation information such as error checks, node identifiers (IDs), fault descriptions, loopback indications, timestamps, etc. OAM cells are identified in the ATM cell header as separate from user cells.
Bellcore Technical Advisory TA-NWT-001248, Issue 1, October 1992, describes on pages 5-12 and 5-13 how Performance Management OAM cells (PM OAM cells), each containing a timestamp, can be used to obtain an estimate of excessive cell transfer delay occurrences at the broadband switching system that receives the timestamp information in the forward report within the forward monitoring cell. It further states that this count can only be made and stored at the connection/segment end point that receives the forward monitoring cell, because at present there is no field in the PM OAM cell that allows backward reporting of excessive cell transfer delay occurrences. Bellcore goes on to state:
If "the clocks of the BSSs are synchronized in absolute time, . . . the one-way delay can be measured directly with a Performance Management OAM cell. However while the frequencies of the BSSs' clocks will be almost perfectly matched in a BISDN network, the absolute time is not expected to be synchronized. In practice, absolute time differences of several seconds are possible.
Whether the clocks are synchronized or not, there is a lower bound on the delays observed at a receiving node. Delays longer than the minimum would be caused by queuing and processing delays. . . . the parameter of interest is how many delay measurements exceed the maximum allowed value, L+V.sub.max, where L is the lowest observed value (obtained through calibration).
When the timestamp is being used, it is encoded in the PM OAM cell at the originating end. This time stamp will be accurate to within .+-.1.0 .mu.sec. The terminating end point compares the time stamp to the time shown by its own clock. This comparison needs to be done as soon as OAM processing has begun on the received PM OAM cell, so that the delay measurement includes as little OAM cell processing time as possible. Variation of the delay experienced by the PM OAM cell will provide a good estimate of the delay variation experienced by the user-information cells.
One can estimate the lowest value, L, by a calibration procedure in which the delays of the first C PM cells C may be e.g. 1000! are observed, and the lowest value is recorded. Note that L may be negative, because the clocks of the two nodes are not necessarily synchronized. The amount by which the observed delay measurements exceed L provides an unbiased estimate of the delay variation."
To summarize, Bellcore states that:
the originating mode must encode time stamps, PA2 the receiving node must calibrate the first C PM cells to calculate L, and PA2 the receiving node must count the number of PM cells with delays greater than L+V.sub.max.
Monitoring can be performed at different locations in a network and the following are examples:
As seen in the above description, the technique described by Bellcore only provides near-end monitoring, and for one parameter only.
In U.S. Pat. No. 5,450,394 Sep. 12, 1995 Gruber et al, good techniques for monitoring delays in telecommunications networks are described. The techniques use measurement cells (test cells, PM OAM cells, test frames etc.,) for sending timestamps between two nodes. In one example in the patent, node A sends to node B a measurement cell containing timestamp value T1 indicating the time the measurement cell is sent, according to a clock at node A. In response to the measurement cell, node B sends to node A a reporting measurement cell containing timestamp value T3 and a delay difference value Td, wherein Td=T2-T1, and T2 and T3 are respectively the times the measurement cell is received at node B and the reporting measurement cell is sent from node B, both according to a clock at node B. Node A then receives the reporting measurement cell at time T4, according to the clock at node A, and calculates delay parameters using T1, T3, T4 and Td.
In the patent, by using these values, node A is able to calculate cell delay variation, (CDV), round trip delay (RTD), cell transfer delay (CTD) etc. The clocks at nodes A and B are not neccessarily synchronized It should be noted, however, that all the above calculations call for T1 value which must be stored at least until T4, that is to say, until node A receives the reporting measurement cell from node B.
The present invention uses similar techniques as described in the patent. The present inventors, however, have realized that for certain delay monitorings e.g., RTD and CTD, timestamp values do not have to be stored at either node.